The present invention regenerally relates to a range finder system and more particularly, to an automatic range finder system suitable for use, for example, in an automatic focusing camera and capable of automatically determining a distance between the camera and a subject to be photographed for focusing. The range finder based on triangular principles includes light emitting means for emitting a light beam toward the subject and an optical system disposed in a position spaced a predetermined base length from the light emitting means, with a plurality of photoelectric or photosensitive elements being provided at an image forming area of the optical system so that reflection of the light beam from the subject is received by the photoelectric elements corresponding to the distance between the camera and the subject to be photographed.
The range finder of the above described type is disclosed, for example, in U.S. Pat. No. 3,723,003, in which, however, construction and function are described only in terms of principles, and no particular techniques that will make it possible to incorporate the range finder into a compact camera for commercial success are disclosed, thus said prior art being too academic to be put into actual application.
Upon investigations into the matter to provide a range finder suited to practical application, the present inventors have obtained findings as follows.
There has been a strong demand for a range finder which can be readily built into 35 mm lens shutter cameras, 110 pocket cameras, movie cameras, etc. without large alterations in size of such cameras, and in the efforts for realizing such a range finder as described above, many new problems have been presented to be solved. Main items required for the range finder of the above described type are, for example, compact size, high reliability, small power consumption, low price, etc.
Incidentally, for enabling the range finder to be incorporated into a compact-size camera, the base length of the range finder is restricted within several cm, and correspondingly, the pitch for arrangement of the plurality of photoelectric elements to be included in the photoresponsive means is also limited within a predetermined value according to the geometric and optical construction of the range finder system. For example, in a range finder system in which the range of distance from 1 m to 5 m between the camera and the subject to be photographed is covered by four photoelectric elements, the pitch between the respective photoelectric elements becomes approximately 130.mu. when the base length is set to be 25 mm, with the width of each photoelectric element being smaller than the above value. Accordingly, if it is intended to limit the size of an image formed at the photoresponsive means by reflection of the light beam from the subject, so as not to exceed the width of one photoelectric element, it is required that the light emitting area of a light emitting element included in the light emitting means is reduced as small as possible, and that the lenses employed for the optical system included in the light emitting means and photoresponsive means should have an extremely small aberration. However, such lenses as described above are generally expensive, and even when lenses with ideal properties are employed, there still remain some problems as described later. On the other hand, in a simple optical system employing, for example, one plastic lens, the aberration is hard to avoid, and even if the light emitting area of the light emitting element is reduced as far as practicable, the size of the image formed by the reflection of the light beam from the subject undesirably exceeds the size to be determined geometrically and optically, thus covering two or more photoelectric elements. In connection with the above, if it is attempted to avoid the image formation extending over the plurality of photoelectric elements by increasing the pitch for the arrangement of the photoelectric elements, the base length has to be undesirably increased, thus going against the intention for the compact size. Meanwhile, it is quite common that subjects lit by surrounding light such as a person brightly illuminated by sunlight, etc. are photographed, and thus, photoelectric elements of the range finder are subjected to incidence of surrounding light apart from the reflection of the light beam, i.e. signal light. As one method for efficiently separating the signal light component mixed with the surrounding light, it is conceived to set the intensity of the light beam emitted from the light emitting means to be also large, but such a practice results in the increase of power consumption and the light emitting area of the light emitting element, and is not suitable for the intended range finder system in which a narrow light beam is required.